Nets and Solids
A solid unfolded is a net
A net is what you get when a three-dimensional object is unfolded and laid out flat, so that every face lies in one plane. Imagine cutting along some edges of a cardboard box and opening it out: the flat shape left on the table is a net of the box. Each face of the solid becomes one region of the net, joined to its neighbours along the edges that were not cut. A net is therefore not a new shape but the same solid seen flat, which is why thinking in nets links the flat geometry of faces to the solid geometry of objects.
The cube and its six squares
The cube is the clearest place to start, because all six of its faces are identical squares. Unfold a cube and you get six squares joined edge to edge, often in the shape of a cross, though several different arrangements fold up just as well. Counting the squares confirms the cube has six faces, and tracing which square meets which shows how the flat net wraps into a closed box. Folding a cube net in the mind, square by square, is the first real act of the spatial reasoning this unit is built on.
Reading a net before it folds
The harder skill is to look at a flat net and see the solid it will become before any folding happens. The clues are the shapes of the faces and how many there are: six squares promise a cube, while one square joined to four triangles promises a square pyramid, and two triangles with three rectangles promise a triangular prism. Reading a net means matching the collection of flat faces to the object whose surface they form. This is reasoning, not memorising, because the same questions answer any net: what faces are here, how many, and how do they meet?
Building the solid from its net
Folding works in the other direction, turning a flat net back into the solid it describes. As each face lifts and turns along its joins, flat regions that sat side by side rise to meet at edges, and the open shape closes into a solid. A child who can build the object from its net, in the hand or in the mind, has understood that the net carries all the information the solid needs: every face, in the right place, ready to fold. This back-and-forth between flat and solid is exactly the spatial and geometric reasoning the curriculum asks for.
Faces, edges and corners
Folding also explains the parts of a solid. Each flat region of the net becomes a face; the joins that the folds bring together become edges; and the points where edges meet become the corners, or vertices. So a cube net of six squares folds into a solid with six faces, twelve edges and eight vertices, and every one of them can be traced back to the flat net. Keeping track of what becomes a face, an edge or a corner stops the parts of a solid from blurring together and makes folding a careful, checkable act.
From net to solid and back
Nets let us travel freely between flat and solid in both directions: unfold any object to read its faces, or fold any net to build its object. Six squares give a cube, a square with four triangles gives a square pyramid, and triangles with rectangles give a prism, each net carrying its solid's full surface. Moving between the two, and naming what each face, edge and corner becomes, is spatial reasoning a child can rely on, and it is the foundation for the volume, surface area and design problems that the rest of the geometry curriculum will bring.